The invention relates to electrical power switching of a load, and more particularly, to determining whether a semiconductor switching device is short circuited in a closed state, and further, to take appropriate action to prevent damage to certain types of loads.
There are many applications where it is desirable to control the amount of average electrical power to a load. One example of such an application is a lamp dimmer. A lamp dimmer typically functions by switching on the ac line voltage to a load (i.e., the lamp) at a point in the half cycle of the ac line voltage between voltage zero crossings, rather than at the beginning of a half cycle. That is, the lamp only receives voltage (is on) for a portion of an ac line voltage half cycle. The greater the portion of the half cycle that the voltage applied to the lamp is on versus off, the greater the light output. As the portion of the half cycle that voltage is applied to the lamp is reduced, the light output is reduced. The lamp load can be a conventional line voltage incandescent lamp, or it can be a low voltage incandescent lamp which is connected to the controlled line voltage waveform through a magnetic transformer.
Conventional lamp dimming circuits may use two (or more) semiconductor switches to control the voltage, and hence, the current and average electrical power to the lamp. One semiconductor switch is for the first, positive half cycle of an ac voltage waveform and the other semiconductor switch is for the second, negative half cycle of the ac voltage waveform. If one of the semiconductor switches fails in the closed state (i.e., the switch short circuits closed), which is the most common failure mode, the light dimming circuit may provide non-symmetrical current to the load. For example, if a dimmer is set to provide 50 percent of full line voltage, and the first semiconductor switch fails closed, the first switch will conduct for 100 percent of its half cycle, and the second switch will conduct for 50 percent of its half cycle. That is, the first switch conducts for the full positive half cycle of the ac line voltage waveform, and the second switch conducts for 50 percent of the negative half cycle of the ac line voltage waveform. As can be appreciated, this type of failure allows more positive current than negative current to flow to the load, thereby creating a non-symmetrical current having a dc component. The dc component may be harmful to some loads. For example, in a magnetic transformer, the dc component of current is dissipated as heat, which may eventually cause transformer failure and/or other damage. The worst case failure typically occurs when the lamp dimmer is adjusted to a minimum and the still normally functioning semiconductor switch provides almost no voltage in its half cycle, and the other semiconductor switch is short circuited. In this case, a load is exposed to half wave rectified pulsating dc voltage.
Therefore, there is a need for a system and method for determining if a semiconductor switch is short circuited in the closed state.
The invention is directed to determining if a semiconductor switching device is short circuited in the closed state.
According to an aspect of the invention, an apparatus is provided for detecting a short circuited semiconductor switch. The apparatus comprises a semiconductor switching device and a short circuited switch detection device. The semiconductor switching device comprises an input, an output, and a control input for switching the semiconductor switching device. The short circuited switch detection device comprises an input and an output. The input of the short circuited switch detection device is coupled to the semiconductor switching device, wherein the short circuited switch detection device detects a short circuit in the semiconductor switch switching device.
According to an aspect of the invention, the semiconductor switching device comprises a first and a second semiconductor power switch in series connection to a common point and a control input that switches each semiconductor power switch. Each semiconductor power switch may comprise a field effect transistor, a metal oxide semiconductor field effect transistor, an insulated gate bipolar transistor with a reverse parallel power diode connected from collector to emitter of each transistor, and the like.
According to a further aspect of the invention, the short circuited switch detection device comprises a first inverter comprising an input and an output, a first resistor, and a first rectifier series coupled to the first resistor. The first inverter is coupled to a drain of the first semiconductor power switch via the series coupled first rectifier and first resistor. The short circuited switch detection device further comprises a second inverter comprising an input and an output, a second resistor, and a second rectifier series coupled to the second resistor. The second inverter is coupled to a drain of the second semiconductor power switch via the series coupled second rectifier and second resistor.
According to another aspect of the invention, the short circuited switch detection device further comprises a third resistor, a first capacitor, and a first zener diode in parallel connection between the input of the first inverter and the common point, and a fourth resistor, a second capacitor, and a second zener diode in parallel connection between the input of the second inverter and the common point.
According to a further aspect of the invention, the apparatus further comprises a full power device that comprises an input and an output. The input of the full power device is coupled to the output of the short circuited power switch detection device, and the output of the full power device is coupled to the control input of the still normally functioning semiconductor power switching device, wherein the full power device enables full power to pass through the semiconductor switching device, responsive to the output of the short circuited power switch detection device.
According to yet another aspect of the invention, a method is provided for protecting a load from a short circuited semiconductor switch in a semiconductor switching device comprising a first and second semiconductor power switch. The method comprises detecting the short circuited power switch, and switching the non-short circuited power switch to full power.